1. Field of the Invention
The present invention relates to manufacture of semiconductor integrated circuits and more particularly to a method of chemical mechanical polishing of conductive layers.
2. Description of the Related Art
Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric interlayers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Copper and copper alloys have recently received considerable attention as interconnect materials because of their superior electromigration and low resistivity characteristics. Interconnects are usually formed by filling copper in features or cavities etched into the dielectric interlayers by a metallization process. The preferred method of copper metallization process is electroplating. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in sequential interlayers can be electrically connected using vias or contacts.
In a typical process, first an insulating interlayer is formed on the semiconductor substrate. Patterning and etching processes are performed to form features such as trenches and vias in the insulating layer. Typically the width of the trenches is larger than the width of the vias. After coating features on the surface with a barrier and then a seed layer, copper is electroplated to fill the features. However, the plating process, in addition to the filling the features, also results in a thick copper layer on the top surface of the substrate. This excess copper is called overburden and it should be removed before the subsequent process steps. FIG. 1A shows an exemplary portion 8 of such plated substrate 9, for example a silicon wafer. As shown in FIG. 1A, vias 10, 12 and a trench 13 are formed in an insulation layer 14, such as a silicon dioxide layer, that is formed on the substrate 9. The vias 10,12 and the trench 13 as well as top surface 15 of the insulation layer 14 are covered and filled with a deposited copper layer 16 through electroplating process. Conventionally, after patterning and etching, the insulation layer 14 is first coated with a barrier layer 18, typically, a Ta or Ta/TaN composite layer. The barrier layer 18 coats the vias and the trench as well as the surface of the insulation layer to ensure good adhesion and acts as a barrier material to prevent diffusion of the copper into the semiconductor devices through the insulation layer. Next a seed layer (not shown), which is often a copper layer, is deposited on the barrier layer. The seed layer forms a conductive material base for copper film growth during the subsequent copper deposition. As the copper film is electroplated, the deposited copper layer 16 quickly fills the vias 10, 12 but coats the wide trench 13 and the surface 15 in a conformal manner. When the deposition process is continued to ensure that the trench is also filled, a thick copper layer or overburden is formed on the substrate 9. Conventionally, after the copper plating, various material removal processes, for example chemical mechanical polishing (CMP), etching or electroetching can be used to remove the unwanted overburden layer. Conventionally, after the copper plating, chemical mechanical polishing (CMP) process is employed to globally planarize and then reduce the thickness of the copper layer down approximately to the level of the surface of the insulation layer.
The CMP process conventionally involves pressing a semiconductor wafer or other such substrate against a moving polishing surface that is wetted with a polishing solution, which polishing solution can also be a chemically reactive abrasive slurry. The slurries are usually either basic or acidic and generally contain alumina, ceria, silica or other hard ceramic particles. The polishing surface is typically a planar pad made of materials well known in the art of CMP. The polishing solution may be flowed over the pad or may be flowed through the pad if the pad is porous in the latter case. During a CMP process a wafer carrier with a wafer to be processed is placed on a CMP pad and pressed against it with controlled pressure while the pad is rotated. The pad may also be configured as a linear polishing belt that can be moved laterally as a linear belt. The process is performed by moving the wafer against the pad, moving the pad against the wafer or both as polishing solution is supplied to the interface between the pad and the wafer surface.
As shown in FIG. 1B, CMP is first applied to reduce the thickness of the copper layer down to the barrier layer that covers the surface. Subsequently, the barrier layer on the surface is removed to confine the copper and the barrier in the vias and trenches. However, during these processes, determining the polishing endpoint, whether the copper layer is polished down to the barrier layer or the barrier layer is polished down to the oxide layer, is one of the important problems in the industry. Typically, in one group of prior art, the substrate is removed from the CMP device and the thickness of the copper layer is measured ex-situ to see if the desired endpoint has been reached. Because, this process interrupts the normal process cycle, it is time consuming and reduces the throughput. Also, the measurements may reveal that the endpoint has been exceeded and the substrate is over polished, which may render the substrate useless. On the other hand, under polishing of the copper layer leads to failure in isolation and causes electrical shorts.
Another group of prior art involves in-situ methods such as electrical or optical methods, or in some cases acoustical methods, to determine endpoint. Most of these methods involve monitoring a parameter associated with the substrate surface and indicating an endpoint when the parameter abruptly changes. For example, one electrical method is to sense the changes in the friction between the wafer and the polishing pad by sensing the motor current utilized by the system or current of the motor utilized by the system. The motor current method relies on detecting the dissimilar coefficient of friction between the polishing pad and the layers that are being polished and stops polishing when a transition is sensed. However, if the overlying and A underlying materials in the polished structure have similar coefficients of friction, sensing transitions from one material to the other becomes difficult. In other examples, optical endpoint detection systems can be used with rotating pad or linear belt systems having a window or windows in them. In such cases as the pad or the belt moves, the openings made therein pass over an in-situ monitor that takes reflectance measurements that are obtained from the wafer surface and reflected through the openings. Changes in the reflection indicate the endpoint of the polishing process. However, windows opened in the polishing pad complicate the polishing process and disturb the homogeneity of the pad or the belt. Additionally, such windows may cause accumulation of polishing by-products and slurry.
U.S. Pat. No. 6,121,147 describes a method in which atomic absorption spectroscopic techniques are used to detect the presence of a metallic substance in the material removed from the wafer. Such techniques are both expensive to implement as well as difficult to operate in a manner that provides real-time results.
U.S. Pat. No. 6,258,205 also teaches an end-point detection method that requires the insertion of an endpoint layer with a catalyst material disposed therein. As a result, when the endpoint layer is removed, the catalyst material will react with a reagent in the solution, and that reaction can cause a detectable change, which can be detected using a sensor.
Each of the above methods thus has drawbacks of one sort or another for end point detection.
Therefore, a continuing need exists for a method and apparatus which accurately and effectively detects an endpoint on a substrate when the substrate is polished using CMP processes.
The above invention overcomes disadvantages mentioned above as one of its objects.
In one aspect, the present invention provides an apparatus for operating upon a multi-layer workpiece using a solution. The apparatus includes a chemical mechanical polisher adapted to remove a top layer from the workpiece, the polisher receiving the solution and emitting a flow of used solution, with the used solution containing therein material corresponding to the top layer that has been removed. The apparatus also includes an optical system adapted to transmit a beam of light on the used solution with a light source and detect an output beam of light with an optical detector, the optical system further adapted to provide an end-point signal to the polisher upon detection of a substantial change in a characteristic of the output beam of light within a predetermined wavelength range due to a substantial change in an absorption of the beam of light by the used solution, the change in absorption of the beam of light by the used solution occurring because of a change in an amount of the material within the flow of used solution, the end-point signal thereby indicating that the top layer has been removed.
In a particular embodiment, the characteristic of the output beam is the intensity.
In a further aspect, the present invention provides a method for detecting removal of a material in a chemical mechanical polishing process that uses a solution and operates upon a top layer made of a material that is disposed over another layer on a multi-layer workpiece. The method includes removing the top layer from the workpiece using chemical mechanical polishing with the solution, the step of removing causing a flow of used solution to result, with the flow of used solution containing therein the material removed from the top layer. While removing the top layer, the method transmits a beam of light on the flow of used solution to obtain an output beam of light that is altered due to absorption by the material, and detects a change in a characteristic of the output beam of light from the beam of light indicative of a change in an amount of the material within the flow of used solution.
In a particular embodiment of this method, the step of detecting the change in the output beam of light generates an electrical signal indicative of the change in the output beam of light from the beam of light, and further including the step of generating an end-point signal to halt the step of removing when the material corresponding to the top layer is substantially reduced within the used solution based upon the electrical signal, the end-point signal thereby indicating that the top layer has been removed.
In another aspect, the invention provides an apparatus for operating upon a multi-layer workpiece using a solution. The apparatus includes a chemical mechanical polisher adapted to remove a top layer from the workpiece, the polisher receiving the solution and emitting a flow of used solution, the used solution initially containing therein material corresponding to the top layer that has been removed and subsequently containing therein another material corresponding to another layer below the top layer. The apparatus also includes an optical system adapted to transmit a beam of light on the used solution with a light source and detect an output beam of light with an optical detector, the optical system further adapted to provide an end-point signal to the polisher upon detection of a substantial change in a characteristic of the output beam of light within a predetermined wavelength range due to a substantial change in an absorption of the beam of light by the used solution, the change in absorption of the beam of light by the used solution occurring because of a change in an amount of another material within the flow of used solution, the end-point signal thereby indicating that the top layer has been removed.
Other aspects and advantages of the invention are described hereinafter, and particularly in the claims.